Process of purifying caustic aluminate liquors



J. L. PORTER 2,889,982

PROCESS OF PURIFYING CAUSTIC ALUMINATE LIQUORS June 9, 1959 `3Sheets-Sheet l 75 MWQZH@ 0H Nm l R. R @m w Y @mw .EN m T W V R R v HEADW. O O UZHUCUMDAUS g @N D., T on L m www mf 2 J MOD/@HA DMHKHKAQU J. l..PORTER 2,889,982

PROCESS OF PURIFYING CAUSTIC ALUMINATE LIQUORS June 9, 1959 3Sheets-Sheet l712 Filed Nov. 25. 1952 :ATTORNEY June 9, 1959 J. L.PORTER PRocEss oF PURIFYINC cAusTIc ALUMINATE LIQUoRs 5 Sheets-Sheet 3Filed NOV. 25, 1952 N0 @W A02 mw O30 GMO 01H02. #N

United States Patent PROCESS .0F PURIFY'KING CAUSTIC ALUMINATE `LIQUORSJohn L. Porter, Baton Rouge,

Aluminum & Chemical JCorporation, -a corporation of Delaware La.,assignor to Kaiser '0akland, Calif.,

This invention in its most ygeneric aspect relates to a method andapparatus for the selective vseparation of two or more solids ofdifferent specific gravities .and/or particle sizes from a liquid oflower density. More particularly the invention relates to a novel systemof centrifuges'and a method to effect the yremoval of precipitates ofmicroscopic and sub-microscopic particle sizes from a viscous liquor,and thereafter to separate the solids of the resultinghigher solidsslurry .into fractions of different specific gravities and/or particlesizes. Specitically, the invention is directed to a novel method andcombination of apparatus for the selective separation of a sodiumcarbonate fraction .from the other precipitates, such as organic salts,which form during the concentration ofcaustic aluminate liquor'utilizedinthe Wet alkali aluminate or wellvknown Bayer process for aluminaproduction.

This application is a continuation-in-part of my copending application,Serial No. 262,808 filed December 2l, 1951 and now abandoned, relatingto a Bayer liquor purification process involving the removal of sodiumcarbonate and organic substances, lwhich latter in the solubilized stateadversely affects the settling characteristics of the red mud or oreresidues from an alkaline digest, -and the precipitation of alumina inthe autoprecipitation phase lof the Bayer process.

The slimy gelatinous nature of the precipitates formed by concentratingthe circulating caustic aluminate liquors creates difficulty ineffecting the necessary separation thereof from the viscous causticliquor. In addition, it is highly desirable to provide a method andmeans of separating the precipitated carbonate from the organic solids,since the former is amenable to direct treatment by causticizing forrecovery ofthe soda as cafustic, while the latter is not. As hereinused, the term causticize refers to the conventional and -Well knownpractices of converting solubilized soda ash or sodium carbonate tocaustic soda by treatment with lime, such as a slaked lime, to forminsoluble calcium carbonate and caustic soda. Conversely,non-causticizable when used with reference to sodium compounds refers-to the inability of lime or slaked lime, when used inthe conventionallime causticizing reaction, to lconvert the soda. values to causticsoda.

This invention is directed to a specific method' and centrifugal systemparticularly applicable in Ithe removal of such precipitates from theconcentrated liquor and the separation of the sodium carbonate solidsandv the organic solids.

The normal circulating 'Bayer process caustic aluminate liquor containsrelatively large quantities of dissolved sodium carbonate and smallerquantities of a complex mixture of `solubilized or dissolved sodiumorganic substances, including salts, which are vformed ,in the causticliquor from `the organic matter, such as humus', present in the ore, and.from the starchwhih yis generally employed -as a flocculating agent insettling .thered mud residue of the ore by the usual decantationoperation.

Patented June A9, 1.9.59

The 4process liquor is progressively lcontaminated with these organicsubstances orsodium organiccompounds (designated ,collectively as.sodium organateS) vand lalso by sodiurn carbonate, the latterrepresenting the so-called process carbonation of the liquor, a term-usedgenerally throughout :the industry. Process carbonation involvesthe vaccumulation of sodiumv carbonate -in the -liquor -from suchsources as .(1) lthe progressive degradation of sodium organatesfof moreor less highermolecular-weight into this `more or less vultimate form,(2) pick-up of carbon dioxide from the atmosphere by the caustic soda,and (3) introduction of soda ash as suchinto the process liquor from.the lime-soda causticizing `reaction when conducted eitherrinside oroutside thevmain liquorfstreaul forproducing make-up caustic. ThissodiumV carbonate, although not fatal to satisfactory mud settling andalumina precipitation, represents an inventory of valuable soda Vin thesystem which must be rectified to caustic soda for use in extraction ofalumina from the ore charges, and to maintain a proper 'balance between'the freecaustic and the total soda in the system.

This contaminating fraction of the total sodium con'- tent Vof theliquor is `denominated herein as `the noncaustic soda since itis notpresent as free caustic soda and therefore knot available to form sodiumaluminate. The fraction of the non-caustic soda which includes theorganic substances or sodium organates is designated herein asnon-alkaline soda, since it comprises sodium vcompounds most of jwhichare not acid titratable. The

l'balancenfzthe non-caustic soda Vis'the sodium carbonate,

'l':E.lattenbeingdirectly causticizable with lime, While the f,-non-allal'inesodalis substantially non-causticizable.

The total-non-alkaline soda, which includes the sodium '.organates, as.well l'as inorganic' sodium compounds Asuch as silicates, phosphates,vanadates, chlorides andsulphates, constitutes a minor 'fraction -of thenon-caustic soda in l VIVthe contaminatedprocess liquor. Nevertheless,it gives rivseto av mosts'erious problem in that at least a portion ofthe v.organic Vcompounds exerts a powerful inhibiting action on thesettling of the red mud ore residues during clarification of therliquor. AIn addition, `it has previously been recognized .that organicmatter in the liquor inhibitsvr auto-precipitation of alumina fromsodium aluminatel liquors. This adverse elect on the settling rate ofthe red mud in the increasingly contaminated liquor is reflected inactualpractice by an -increased starch consumption for equivalentsettling results. However, increasing starch feed rates aggravate theproblem by increasing the net concentration of organic matter as sodiumorganates (non-alkaline soda) in the liquor, since the starchdegradation products, as well as sodium organic compounds from theorganic matter `in the ore, include potent settling inhibitors.

Of vital importance in the necessary production of alumina from thelower grade aluminous ores, is the elfect of the settling "inhibitorportion of the non-alkaline soda (sodiumorg'anates). Most of the lowergrade ores, such asythefWes "ln 'n bauxites and laterites, contain largefractio'nspofY ,residues (hydrated ferrie oxide) of a highlydispersib u,slowqor substantially nonf'settling nature. vlt/Ioreover,"these lowergrade ores contain higher amounts of organic matter, such as humat andthe like, .than the high grade tr'ihydrate ores. Thus, theynonalkalinesoda vconcert'tration..of"the circulating process liquor builds up to `ahigher` level. This combination of factors actually 'leads'tfthe resultthat no practical settling rate for the 'red mudcan be obtained withoututilization of excessive quantitiesV of starch. As indicated above,excessive-starch consumption 'increases the concentration of`non-alkalirvle soda', including settling-inhibiting sodium. organates,in the'recycle liquor through degradation of `the starch and the netelect after repeated recycling of the liquor is negative on the red mudsettling rate and a further inhibition of the auto-precipitationreaction. Accordingly, ecient processing of lower grade aluminous oressuch as the Jamaican type bythe Bayer process Without control of thedeleterious sodium organates (nonalkalinc soda) is.not attainable. Theinvention of my copending'application S.N. 262,808 and now abandoned isdirected to a particular manner of effective control of processcarbonation and recovery of the sodium carbonate, while concurrentlysubstantially freeing the liquid of settling inhibiting sodiumorganates.

-As described in my copending application the problems of settling andauto-precipitation are overcome by concentrating the contaminated Bayerliquors to obtain a slimy gelatinous precipitate containing sodium saltsof organic substances as well as sodium carbonate, and recovering theprecipitates from the concentrated Bayer liquor. The precipitates of thesodium organate compounds are extremely ne, and in fact a portion of thesalts thus formed during concentration by evaporation are colloidal innature and accordingly practically impossible to separate from themother liquor by conventional methods. On the other hand, the sodiumcarbonate forms as crystals which are more easily separated fromthe'mother liquor together with a substantial fraction of the sodiumorganate salts. The substantial separation of precipitated salts fromthe concentrated liquor is critical to the substantially completerealization of the objective of removal of the organic settlinginhibitor compounds which may be contained therein with the consequentincrease inmud settling rate and starch effectiveness, and to theobjective of control of process carbonation.

It has been discovered that the separating of the gelatinous precipitateslurry from the liquor may best be accomplished by continuouscentrifuging, the gelatinous nature of certain of the organicprecipitates making liltration or decantation a practical impossibility.This is especially-true in view of the large volumes of liquornecessarily handled in order to remove the relatively small amount ofprecipitate which forms during the liquor concentrating operation.

i In general according to the process of my copending application, theconcentration of the liquor should proceed to at least 350 grams per'liter caustic soda (as lequivalent Na2CO3) when confronted withnon-caustic `soda in 'what may be termed normal amounts, although theparticular. caustic soda concentration at which the gelatinousprecipitate is obtained varies necessarily with the' level orconcentration of the non-alkaline soda impurities in the liquor (and tosome extent with the varying specic composition of these impuritieswhich is relatively unknown and indeterminable). The amounts of both theslimy. gelatinous precipitate of the complex mixture of sodium organiccompounds and the sodium carbonate precipitate may be increased byincreasing the caustic soda concentration up to about 600 grams perliter. However, the density of the viscous caustic liquor is increasedat such high concentration and tends to increase the dificulty inseparation of the sludge therefrom. In regard to specific conditions ofliquors investigated, it has been found that caustic soda concentrationsof from about 380 to about 500 grams per liter were most advantageous.

Of importance in the method of separating the sludge containing thesettling inhibiting organic compounds precipitated by the liquorconcentration or salting out process isthe method of recovering the sodawhich is bound in the compounds thus precipitated.

One method of recovering the soda is to mix the sludge separated fromthe concentrated liquor with bauxite or other alumina containingmaterial, including red mud residues obtained from digestion of highsilica containing ores, and subject the mixture to a sintering operationat appropriate temperatures, for example, 1,000 to 1,200

degrees C., preferably 1,050 to 1,150 degrees C. (or about 1,900 to2,100 degrees F.) to destroy the organic matter, thus decomposing thesodium organates and also the sodium carbonate to form sodium aluminatewith the alumina in the bauxite. Lime is also added in proper amounts tominimize the reaction of soda and alumina with the silica in the ore andsubsequent loss in silicate form. The product is then leached with adilute caustic soda solution or with dilute spent liquor and the leachliquor obtained corresponds to a green caustic aluminate liquor readyfor precipitation. In this manner most of the alumina in the bauxite oraluminous material and the mother liquor is recovered, together with asubstantial portion of the soda as sodium aluminate while most of thesoda is also recovered as caustic soda, thus increasing the caustic sodaconcentration of the leach liquor.

Sodium carbonate constitutes on the order of of the precipitate formedduring the salting out operation, and this fraction may be directlycausticized with lime to form caustic soda and insoluble calciumcarbonate. Accordingly, it is highly desirable to separate the sodiumcarbonate fraction of the sludge from the non-causticizable fraction ofthe precipitates containing sodium organates in order to decrease theload on the sinter operation. Accordingly, as an alternative to thesinter method of recovering the soda a fractionation of the precipitatedsludge may be accomplished to give:

(1) vA high solids slurry rich in sodium carbonate, low inalumina-containing mother liquor, and low in the slimy gelatinousprecipitate of sodium organates or non-alkaline soda compounds.

(2) A low solids slurry containing both sodium carbonate andsubstantially all of the non-alkaline soda solids of the sludge 4andrelatively high in mother liquor containing alumina.

The carbonate rich-low alumina, low non-alkaline soda slurry isadaptable to direct causticiziug to rectify the sodium carbonate tocaustic soda. By recovering a substantial portion of the soda in lthismarmer the amount ofV soda recovered in the sinter of the remainingsludge is appreciably reduced with an overall monetary saving in theoperation as compared to a sinter of the entire sludge. It is with thisparticular method of liquor clari fication and sludge :fractionationthat the present invention is concerned.

It has been discovered according to the hereinafter described inventionthat the causticizable sodium carbonate fraction of the precipitatesformed during concentration of a Bayer plant liquor may be separatedsubstantially yfrom the remaining organic salts of a substantiallynon-causticizable nature by subjecting the total slurry to separation ofthe solids en masse from the concentrated caustic Iliquor by means of ahigh speed line centrifugal separator, as will be more fully explainedsubsequently; and thereafter subjecting the `resulting thickened solidsslurry to the action of an intermediate speed centrifuge of the coarsecentrifugal separator type for the selective separation of the sodiumcarbonate from the sodiumorganates. Sodium organates as herein usedrefers collectively to the sodium oxalate and other organic non-alkalinesoda `as hereinafter deined. The invention further provides asubstantially completely claried concentrated caustic liquor for returnto the main Bayer plant system.

Accordingly, it is a primary object of the present invention to providea centrifuge system which will not only substantially clarify theconcentrated caustic aluminate liquor of the diicultly separablegelatinous sludge, but which will selectively separate the limecausticizable component substantially from the non-causticizablecomponents.

' VAnother object of the 'invention is to provide a method for theprimary separation of the sodium carbonate and organic precipitates fromthe caustic aluminate mother liquor followed by the selective separationof the organic assaesa precipitates from the major `portion Iof .thesodium carbonate.

Furthermore, fit is au object of the invention toprovide a centrifugesystem and method for the removal of a solids sludge containingparticles of near or substantially colloidal size from a viscous liquorwherein-the solids are of different specific gravities and/ or particlesizes and the liquor density is substantially less than that of thesolid of least specific gravity, and thereafter separating a `fractionof the solids of higher specific gravity and/ or particle size from thesolids of lower specific gravity and/ or particle size.

Other objects will become apparent from the following kdetaileddescription of the invention illustrated in its preferred embodiment asapplied to separation and fractionation of precipitated sludge formed onconcentrating a Bayer process caustic liquor, but without limitation ofthe scope of the invention thereto.

As is well-known in the centrifuge art, the characteristics of aparticular centrifugal separator and the method of operation define thelimits regarding separating power and through-put. Thus, there `arecentrifuges available capable of handling large volumes of solidscontaining liquor which operate with a high through-put. Thisnecessarily leads to a more or less incomplete separation of the solidsand consequently .this type of centrifuge, examples of which are theBird type, such as fully disclosed in lU.S. Patents Nos. 1,710,315 and1,962,461, has been principally employed for scalping operations, forexample, the removal of coarse particles, such as sand, prior to thefurther processing of the overflow and/ or prior to more completeremoval of the solids by other means such as filters orother types ofcentrifuges. In general, this type of apparatus has been labeled acourse centrifugal separator. It is yto be understood rthat in thisspecification the use of the term coarse centrifugal separator refers tothe above type of apparatus lwhich is chiefly characterized in that themechanical rremoval of solids is by a screw conveyor rotating relativeto the centrifuge bowl or rotor and removing the solids generallycounter to the force of Igravity exerted by the centrifuge operation.This type apparatus will be more fully defined hereinbelow withreference to Figure l.

Also well known in the art are centrifuges having a low through-put incomparison to the coarse centrifugal separators. This type of apparatusis characterized by the discs :and discharge orifice nozzles whichcontribute to a high degree of clarity in the overflow and are oftenreferred to as fine centrifugalseparators, examples lof which are thecommercially available Sharples DH-Z Nozzle Centrifuge and the De LavalAC-VO Centrifugal Separator. Accordingly, it is to be understood thatthe mechanical fine centrifugal separator as used herein has referenceto the above type centrifuges.

With respect to either type of machine a decrease in clarity necessarilyfollows an increase in through-put with other operating factorsconstant. Moreover, a higher degree of clarity maybe obtained at thesame throughput by increasing f the -r.p.m. and consequently the numberof multiples of' gravitational force to which the liquor xis subjected.These factors are common operational and engineering factors not formingIa part ofthe invention.

It has been discovered according to the invention that substantially allof the sodium carbonate may be removed from the concentrated viscouscaustic aluminate liquor and a high degree of removal of the remaininggelatinous solid precipitates obtained by subjecting thesludge-containing liquor to the action of a fine centrifugal separator.Moreover, it has been discovered that by thereafter subjecting the moreconcentrated or thickened solids slurry obtained from the finecentrifugal separator to the action of a coarse centrifugal separatorsubstantially complete removal of a sodium carbonate fraction from thesolid sodium organates or other precipitated material may be obtained. Amajor advantage of this discovery is that a causticizable'product maybe.se-

cured substantially kfree of non-causticizable solids with a resultantdecrease 'in the sinter requirements for the recovery of the `sodacontainedin the precipitated slurry.

The invention will be more'fully understood'with reference to theaccompanying figures and tables.

Figure l is a schematic diagram showing the preferred system ofthe fineand coarse centrifugal separators of the herein described invention.

Figure 2 is a schematic flow diagram showing one embodiment by which thesludge in the yconcentrated liquor is separated by selectivecentrifuging into separate fractions, one constituting causticizer feedand the other constituting the feed to the sinter operation asindicatedin Figure 3.

Figure 3 is an overall flow sheet covering the salting out,centrifuging, causticizing, sinter and leaching system according totheconditions shown.

Referring now to Figure l, there is provided two primary finecentrifugal separators operated in parallel so as to `remove en massethe total sludge, that is, both the sodium carbonate and sodium organatesolids from the viscous mother liquor to the desired concentration ofsolids for the feed to the coarse centrifugal separator wherein theslurry containing precipitates of both sodium organates and sodiumcarbonate is subjected to separation into a fraction of the sodiumcarbonate component and a fraction containing substantially all of thenoncausticizable soda solids.

4, 5 and 14 represent thefine centrifugal separators of the disc typehaving orifice type discharge nozzles. Speeds of the order of 6,000r,p.m., thus developing about 9,000 g.s at a maximum bowl diameter of 18inches are common with this type of apparatus although by `no means alimting feature of the apparatus as applied to this invention.

11 is a coarse centrifugal separatorof the Bird type developing about255 gs at an 18 inch bowl diameter for about 1,000 r.p.m. The coarsecentrifugal separator 11 comprises a hollow rotor 21 mountedin astationary housing or frame 20. The rotor is provided with hollow shafts25 at opposite ends which are supported in the stationary housing 20bysuitable horizontally Aplaced bearings `(not shown). The rotor 21contains a centrifugal separating chamber 22 containing a beach 30 ofinert material which retains its vposition during operation thus causingthe effective separating chamber 22 to taper from the large end 32 tothe small end 33. It is to be understood that some models of the coarsecentrifugal separators comprise a rotor which is tapering itself,following the contours defined by the beach at the surface where v thesolids separate out and accordingly-such apparatus is also included inthe specification and appended claims when a coarse centrifugalseparator is referred to. In the large end 32 of the separating chamberare openings 23 forming an outlet for the overflowing liquor and solidsthat have not settled out of the liquor. At .the small end of the rotorare openings 24 for the vsolids discharge located at a smaller radiusthan the opening 23 for the overflow. The periphery of the effectiveseparatingchamber between ends 32 and 33 is frusto-conical in form andforms Within the chamber a path for conveying the separated solids tothe sludge outlets 24.

A cylindrical distributor 27 is mounted in the rotor 21 and is rotatablerelative thereto on the same axis as the rotor. This hollow distributor27 has hollow shafts 34 which support the distributor 27 within thehollow shafts 25 which in turn support the rotor 2l. The distributorshafts 34 and the surrounding rotor shafts 25 extend through openings 35defined by the housing 20 at opposite ends and are arranged to be drivenso that the rotor 21 rotates at a somewhat higher speed than thedistributor 27. An example of the differential drive arrangement shownin Patent No. 1,962,461 issued to E. G. Piper.

Feed tube 26 extends through the hollow shaft"3,4 ofV the distributor 27atthe larger end and into the distributor 27 itself and forms an inletfor the mixture to be separated. f The discharge end of the feed tube 26terminates in a chamber 36 having peripheral openings 28 through whichthe mixture from the feed tube 26 passes outward under centrifugal forceinto the separating chamber 22. The distributor openings 28 arepositioned to deliver the mixture to the tapered periphery of theeffective separating chamber at a region where the periphery of theeifective separating chamber 22 is at approximately the same radius asthe eiuent outlets 23.

A conveyor 29 is mounted in the separating chamber 22 and is rotatablerelative to the rotor 21. As shown, the conveyor is in the for-m of ahelix or screw and is secured to the distributor 27 in any suitablemanner, as by means of connections 37 so that the screw conveyor 26rotates with the distributor 27. The outer edge of the conveyor 29 isclosely adjacent the tapered inner surface of the beach 30 and theconveyor extends from the larger end 32 of the effective separatingchamber or Zone to the smaller end 33. Since the conveyor screw rotateswith the distributor at a lower speed than the rotor 21, it is adaptedto transport separated sludge inward along the tapered surface of thebeach 30 to the sludge outlets 24.

The stationary housing 20 has `a collecting chamber 38 which receivesthe sludge discharge through outlets 24. The housing also as acollecting chamber 39 which receives the eiiiuent discharge throughoutlets 23.

The method of the invention may be described with reference to thecentrifugal system of Figure l as applied to a precipitated sludge ofsodium carbonate and gelatinous organic substances, probably of sodiumsalt character obtained by concentrating a Bayer plant spent liquor. Theunderow from the salting evaporator containing both sodium carbonate andsodium organate salts (nonalkaline soda solids) in the form of a viscousgelatinous sludge is fed to the primary separation system through pipe 1and branch pipes 2 and 3. As shown two ne centrifugal separators 4 and 5operated in parallel comprise the primary separation system. However, itis t be understood that any number may be used depending on the capacityof the respective machines and demands of the operation. Substantiallyall of the sodium carbonate and about one-half or more of the sodiumorganate solids are separated during the primary separation and are ledfrom primary separators 4 and 5 by means of underflow pipes 8 and 9wherein they are combined in pipe 10 to form the feed to the selectivecentrifuge 11.

The substantially clarified liquor overows through spouts 4a and 5a onprimary separators 4 and 5, respectively into pipes 6 and 7 for returnto the main liquor stream. Depending on the degree of clarity demandedby the operation, the liquor in the overflow from the line centrifugalseparators may be further clarified by suitable means not shown ifnecessary.

The underow from the primary centrifuges is combined in pipe 10` and fedto the coarse centrifugal separator 11 wherein the selective separationof the solid sodium carbonate from the sodium organates takes place. Theslurry enters feed pipe 26 and is discharged into chamber 36 of thehollow distributor 27 which discharges the slurry into the separatingchamber 22 through peripheral openings 28. The solid sodium carbonateseparates from the mother liquor containing the sodium organate solidsalong the beach 30 by centrifugal force forming a carbonate sludge 31which is conveyed from the chamber by means of the screw 29 through port24 to the solids collecting chamber 38. Thereafter, the solid sodiumcarbonate is transported to the lime causticizing unit by suitable means12. The solid sodium organates are not separated from the mother liquorduring the coarse centrifugal separation and accordingly collect at thelarge end of the eifective separating zone 22 and overflow through ports23 into the overflow collecting chamber 39 wherein they arercollectedVand transported by means of pipe 13 to the secondary fine centrifugalseparator 14. Herein the non-alkaline soda solids are concentrated inthe underow and led by means of pipe 15 to the sinter unit. Thesubstantially clarified liquor overows into the overow spout 14a and isreturned to the system through pipe 16. Like the overflow from theprimar-ys, the overow from the secondary 14 may also be furtherclarified where deemed necessary.

Solids recycle pipes 17, 18 and 19 are provided on primary iineseparators 4 and 5 and secondary fine separator 14, respectively forcontrolling the solids content in the discharge from the orifice typenozzles as is well known in the art.

Referring to Figure 2 there is shown a schematic flow diagram of oneembodiment of the invention by which the sludge in the concentratedliquor is separated by Iselective centrifuging into the two separatefractions, one constituting causticizer feed and comprising principallysodium carbonate and other constituting the feed to the sinter operationand containing substantial amounts of the remaining organic precipitatesas indicated in Figure 3.

A concentrated Bayer process spent liquor having the analysis asindicated in Figure 2 and containing the precipitated sludge in theamount of about 3% solids by weight is charged through line 50 to a highspeed disctype line centrifugal separation system 51 where the sludge isconcentrated to about a 25% solids slurry discharged as underflow at 52and the clarified liquor containing only about 0.5% solids by weight isrecycled to the mainline spent liquor through line 53. The partiallyconcentrated sludge is introduced into a centrifuge of the solid bowltype or coase centrifugal separator 54 wherein it is centrifuged atintermediate or moderately slow speeds to produce an underflow 55 ofabout 50% solids by weight containing substantially all of the solids assodium carbonate as indicated in Figure 3. This constitutes the feed tothe causticizer unit. This high solids slurry advantageously contains aminimum amount of alumina having been thickened to a high solids contentso that the loss of alumina as calcium aluminate upon reaction with limein the causticizing unit is reduced as far as possible. The overflowfrom the bowl type centrifuge containing about 10% solids by weight anda major portion of the non-alkaline soda or sodium organate compounds isintroduced through line 56 to another high speed line centrifugalseparation system 57 whereinthe lsludge is concentrated to around 20%solids slurry (by weight). This underow 58 constitutes the feed to thesinter operation set out in Figure 3. The overlow from centrifuge 57 ispassed through line 59 to the mainline spent liquor system along withthe overiiow from the centrifuge system indicated at 51, both of whichcontain only about 0.5% solids by weight.

Now referring particularly to Figure 3, there is shown the overallrelation of the centrifuge system of the present invention to thepreceding salting evaporation, and subsequent causticizing, sinteringand leaching systems of my copending application Serial No. 262,808 andnow abandoned. A selected fraction of contaminated spent liquor to bepurified having the analysis lndicated on the ow sheet is introducedthrough line 60 into a salting type evaporator 61 wherein the liquor isconcentrated to a caustic soda concentration of 450 g./l. to obtain aslimy gelatinous precipitate of a portion of the non-alkaline soda or`sodium organates and a precipitate of sodium carbonate. Theprecipitated sludge of non-caustic soda compounds constitutes 3% byweight of the salting evaporator effluent.

The effluent liquor from salting evaporator 61 is passed through line 62to the centrifuging operation indicated at 63 where the sludge isconcentrated and separated from the concentrated liquor to produce ahighly clarified liquor containing only 0.5 solids by weight which isrecycled to the mainline spent liquor as indicated. The

@assunse thickened sludge, as shown in Figure 2 isseparated by ,thecentrifuging operation into twofractions as separate centrifugedunderows. One fraction of thissolids content, namely 50% solids byweight containing 95.8% of its solids as sodium carbonate, is subjectedto direct causticizing by passing the slurry through line 64 to thecausticizing unit 65. Only a very low content of noualkaline soda ispresent in the sludge of this slurry and also in the liquor associatedwith the sludge. As aconsequence, the non-caustic soda content of theslurry is substantially all in the form of the readily causticizablesodium carbonate with a minimum content of non-causticizable ordifliculty causticizable non-alkaline soda compounds or sodiumorganates. This liquor is diluted by Lthe addition of water as indicatedat 66 to adjust the caustic soda concentration of the liquor in thecausticizing unit to a low value which will permit approximately maximumefficiency of the causticizing reaction conducted therein, that is, notless than about 90% conversion of the sodium carbonate charged to theunit. The feed to the causticizing unit may be fortified by the additionof fresh soda ash as indicated at 67 in the flow sheet. The lime forcausticizing is charged at 6 8 into the'causticizing unit with 1.05 molsof lime beingchargedufor each mol of sodium carbonate in the liquor tobe causticized. The causticizing operation is conducted at a total sodaconcentration of 150 g./l. at the atmospheric boiling point of theliquor and produces a causticizer effluent asindicated at 69 having acaustic soda concentration of 135 g./l. and a total soda concentrationof 150 g./l. indicating that the causticizing unit operates at aconversion factor of about 90%. The caustic soda to total 'soda ratio inthe causticizer elinluent liquor will v'ary'somewhat depending upon therelative proportionsvof soda ash contributed by the centrifuge underflowslurry and by the fresh soda ash. For example, with 100% soda ash feed aC/S ratio of 0.953 has been obtained while` witha 50% soda ash-50%centrifuge Vunderflow Vfeed to the causticizer a C/S ratio of 0.923 hasbeen obtained.

The second fraction of separated precipitated sludge thickened to a 20%by weight vsolids content contains 81.5% of the solids assodiumcarbonate with a high concentration of 18.5% of its solids asnon-alkalinesoda, that is, sodium organates precipitatedfrom the liquorby concentration. This fraction is passed through line 70 "to .a sinteroperation 71 wherein it is mixed with bauxite orey in amount to giveabout .908 mols of lalumina in the mixture per mol of soda in the sludgeslurry from line 70. A lso, lime is added in the amount of 2.1 mols permol of silica in the ore charged to the sinter roperation in order tominimize loss of alumina from' the bauxite and from the sludge asinsoluble'sodium alumil num silicate. The sinter charge adjusted to theproper solids content is heated to a temperature approximating 1,950degrees F. after whichv the sintered product is charged through line 72to a leaching system. The sinter product is leached at 73 by theaddition of causticizer eiuent liquor through line 74 diluted at 75 withwater. In the sintering operation substantially all of the non-causticsoda of the sludge is converted to sodium aluminate which is dissolvedin the leaching system 73 by the diluted causticizer efuent liquor toproduce a leaching eiiiuent 76 which is a sodium aluminate lenrichedcaustic liquor having an alumina to caustic soda ratio of 0.60 and acaustic concentration of 170 g./1. The leaching system eiuent is alsovfortilied in caustic soda from the'soda produced in the sinteringoperation in excess of the amount combined with the alumina of the ore.This liquor corresponding to Bayerprocess pregnant -liquor may then becycled to the main plant precipitators for recovery of the alumina afterseparation of `the insoluble residue `as at 77 while the causticizer@lisent iS 90m12.111s@ with the clariiedccncentratcd caustic liquor fromthe centrifuge operation and the untreated spentliquor `for recycling-tothe main plant digesters ofthe continuous Bayer process.

The table hereinbelow isa material balance based on analytical resultsobtained during a typical run of the herein disclosed method ofclarifying concentrated Bayer plant liquors of the solids and thereafterseparately recovering the solid sodium carbonate and the solid organicsubstances (non-alkaline soda). The'iparticular operatingconditions uponwhich the material balance is `based `are as follows:

Eeed rate to salting evaporator 135 gpm.

Liquor temperature in evaporator 255 F.

Feedrate to primarys 42 gpm/centrifuge.

Number of primarys 2. Maximumgs developed 9,800.

Feed rate to selectiveV separatorgm 18.3 g.p.r n.

Number of separators 1. Maximum gs developed 1,020. Feed rate tosecondary 15.45 g.p.m.

Number of secondarys 1. Maximum gs developed 9,800.

The following methods of analysis andreporting were used as a basis forthe material balance:

A1203 (alumina): This is a measure of the alumina as determined bycarbonation and reported as corrected -for 0.8 of the Si02 and all oftheP205 and V205.

C.S. (caustic soda): This is determined by titration on `a standardprecision scientific titrometer for thefree caustic and 'reported withthe calculated caustic bound with the alumina. 'i

Carbonate soda: This is determined by standard alkalimetric methodsbased on C02 evolved.

35 (tota-l soda): This is the sum ofthe caustic soda and carbonate soda.

T. --N a (total sodium): This is determined by the stand- Yard uranylzinc acetate method of measuring the total sodium ion present.

Oxa-late soda: This is determined by a perchlorate-serate titration andis a relative measure of the amount of soda bound with oxalate radicals.

Inorganic non-allcaline soda: This is the sum of the soda bound withsilicate, phosphate, vanadate, chloride and sulphate. The Si02, P205 andV205 are determined by standard colorimetric methods. The chloride isde. terminedby a precipitation titration with AgNO3 and the sulfate isdetermined in standard gravimetric meth; ods withbarium sulfate.

Total non-caustic soda: This is the diiference between the total sodiumand thecaustic soda. Total non-alkaline soda: This is the differencebetween the non-caustic soda and the carbonate soda. v Organicnon-alkaline soda: This is the diiierence between the total non-alkalinesoda and the sum of( the oxalate sodafand inorganic nonalkaline soda.

The total vilo`w`is based on actual weights whe-reas the soda valuesvare'reported as equivalent`Na2CO3.

The table linearly"indicates the degree of separation obtained inf'thefprimary iyi-nevcentrifugal separators for this particular operation. 0fthe total solids fed to the primarys im .2030 are separated into theprimary underflow which ultimately is the feed to the selectiveseparator. Of the solid sodium carbonate formed in the salting operationY v is ser Hated and 0f *he total non-amalis@ Salts farma@ is found inthe primary underow. Moreover, since the 11 total non-alkaline termcontains organic, inorganic `and oxalate soda,

feed, the throughput, size of the bowl and/or gs (multiples of gravity)developed are the most influencing. Thus, besides the particularoperation of the coarse centrifugal g-X l=61.5% jseparator, priorprocessing of the solids to a proper con- 5 centration in the feed is ofcontributing importance. of this is oxalate soda and The hereindescribed apparatus and method of opera- 0031 .tion may alsoadvantageously be used in other clarificaa WX 100= 18-65% tim alriselectztife sepaiaon operatioirlis wherein mixtu/res o Vsoi ar 1c es o 1eren s eci c gravi ies an or 1S rgamc Soda or about 80% of thenomillkalme Soda 10 particle sile may be separated fom a viscous liquorof Sohds recoverd are Qrgamc compounds Whlch dversely lesser, density,and the solids then fractionated. The Ieeclimmg of the red mud and/0ralumma auto' main advantage in using this arrangement lies in the fact'y 1 that fewer 'ieces of a aratus are re uired to et the The Selectlveseparatory functlon of the coarse c enm-f' desired classliication rests.Thus, if afoarse cengrifugal ugal Separator 1S clearly brought out13S/.examination 9i l5 separator is used as a scalper of the higherdensity solids the Ovefow and underfw of the-se1ectwe Separator m in therst instance, the load on the subsequent fine cen- Companson FO the feed.1"6" the pnfnaly underflow' of trifugal separation is not diminishedbecause of the relathe total solld non-,caustic soda which includes boththe tively high amount of liquor going out the Overflow of Carbonatesoda and total fwn-alkalme soda removed m the coarse centrifugalclassifier. Moreover, the number the undercw of the Selectwe Separatorof coarse centrifugal separators will necessarily be in- 0933 100 96 7,7creased when they are used prior to the fine centrifugal .0967 oseparators for the same reason, i.e. the lgreater throughis carbonateSoda Whereas my put requirements on a lower solids content liquor.Accordingly, although the invention is in no way limited to '0557X100=80,8% 25 the number of primary fine centrifugal separators, or'0690 intermediate coarse separators, in its generic aspect it of thesolids in the overow is carbonate soda. The contemplates a centrifugalsystem particularly applicable underflow of the selective separator isthus substantially to a low solids slurry as feed to a primary highspeed all readily lime causticizable Na2CO3 whereas about 20% 30centrifugal clarication and thickening, followed by an gf the solids inthe overow are non-alkaline soda comintermediate coarse centrifugalfractionation of the solids pounds not readily causticizable with analkaline earth in the thickened primary underliow, and high speed orhydrate from which the bound soda may be rectified to fine centrifugalclarification of the coarse centrifugal caustic by the sinter method.separation overflow.

TABLE I Basis: Pounds of component per pound caustic soda to evaporatorPrimary Selective Separator Secondary Evalp. t331s- C ar e OverowUnderdow Overflow Underfiow Overow Underrlow Liquid solid Liquid soildLiquid Solid Liquid solid Liquid Solid Liquid solid Liquid solidTotalluow 3.2250 .2030 2.5533 .0395 .6718 .1635 .0142 .0654 .0575 0980.3828 .0005 .2315 0640 Ahoi .3200 .2581 .0079 .0621 .0058 .0887 .0234o.S. 1.0000 .7910 .2084 .1906 .0178 .1187 .0718 T.s. 1.1014 .1520 .8718.0028 .2295 .1492 .2099 .0557 .0100 .0935 .1308 .0004 .0791 .0553 T. Na1.2607 .1884 .9981 .0226 2620 .i058 .2401 .0090 .0225 .0007 .1490 .0055.0905 .0030 oxalate Soda .0195 .0200 .0159 .0157 .0041 .0102 0037 .0088.0003 .0014 .0023 .0038 .0014 .0051 Iuorg.Non-A1k. soda-- .0235 .0008.0180 .0035 .0049 .0038 .0045 .0010 .0004 .0017 .0028 .0005 .0017 .0012org. Nou-Alk. Soda.- .0036 0923 .0006 .0241 .0031 .0220 .0029 .0021.0001 .0137 .0008 .0083 .0021 Toia1Nou-A1k-soda. .0304 `i263 .0198 .0331.0106 .0302 .0133 .0028 .0032 .0188 .0051 .0114 .0084 Carb. soda .1520.0302 0028 .0211 .1492 .0193 .0557 .0018 .0935 .0120 .0004 .0073 .0553TotalNon-Gaus. Soda .1884 2005 .0226 0542 .1058 .0495 .0000 0040 .0907.0308 .0055 .0187 .0030

- Although the data presented in the above table is I claim: based onone set of operating conditions it is not to be Y 1. A process forclarification of viscous concentrated construed 3S limiting the SCOPe 0fthe HVeDOD l any caustic liquors containing finely divided solids ofdifferent Way but father S Presented merely aS al1 eXaIIlPle 0f 011especific gravities and particle size and for fractionating embodiment 0fthe inVentOn- Higher Speeds and OWef the solids content, the liquorhaving a density substanthrough-Put fesultill a gfeetefepafafio in EhePrimary tially less than that of the solid of least specific gravity,stage of the centrifuge OperatlOIL which comprises mechanicallyseparating substantially F01' the Selective Separaefy OPefaflOn @leydesldeatum all of the solids from the liquor by high speed ne cenls t0Obtain an undljvw 1118151111 SOhdS Whlc Colleen S0- uifugaiclarification, subjecting thc thickened solids undlulfn Carbonate Wltn amI-mmum amounf 0f SOlds as derflow slurry of the fine centrifuging to anintermediate Sodium Organates (Oxalate Soda and Orgamc nou'ahalne 65speed coarse centrifugal mechanical separation to pro- OmPPPndS) bcaus?these Compounds ,are not reamly duce a more concentrated underowfraction of solids Caustlczagle "1m-1u1 ang I )ecuse the 1%? Causucconsisting substantially entirely of the relatively higherngcnbrcioiq'iu nlecn, iini. Specic gravity and ,relatively largerPaftidezef and a liquor stream wherein their adverse effect on settlingand iess concentrated Solfds Oemjv slurry Contauung a ma' autoprecipitaton is once again realized Accordingly, jor portion of thesolids of relatively lower specific gravity for each Particularapparat-Of ,the type labeled coarse and relatively smaller particlesize, and subjecting the centrifugal separator there is an optimummethod of Overflow from the coarse centrifugal separation to highoperation wherein the high sodium carbonate underflow Speed linemechanical centrifuging to substantially commay be obtained. In generalthe amount of solids in the pletely clarify the liquor and to recover amore concen- 13 trated slurry of solids of relatively lower specificgravity and relatively smaller particle size.

2. A process for the clarication of concentrated caustic aluminateliquors containing nely divided solids of the sodium carbonate andsodium organic compound types, and for selective separation of thesodium carbonate from the sodium organic compounds, which comprisesmechanically separating substantially all of the solids from the liquorby high speed ne centrifugal clan'- fcation, subjecting the thickenedsolids underflow slurry of the ne centrifuging to an intermediate speedcoarse centrifugal mechanical separation to produce a more concentratedunderow fraction of solids which is substantially all sodium carbonateand a less concentrated solids slurry as the overflow fractioncontaining the predominant portion of the sodium organic compound typesolids present in the underow slurry from the fine centrifuging, andsubjecting the overow from the coarse centrifugal separation to highspeed ne mechanical centrifuging to substantially completely clarify theliquor and to recover a more concentrated slurry of sodium organiccompound type solids.

3. In a process for the clarification of concentrated caustic alumnateliquors containing a precipitated solids content of a gelatinous naturecomprising lime causticizable and non-causticizable salts, the method ofseparating the lime causticizable from the non-causticiza-ble salts,which comprises subjecting the low solids liquor to high speed nemechanical centrifuging to substantially completely clarify the liquoras overow and to produce a more concentrated solids slurry as underflow,and subjecting the thickened underow slurry of the iine centrifuging toan intermediate speed coarse centrifugal mechanical separation toproduce a more concentrated underflow fraction of solids which issubstantially all lime causticizable salts, and a less concentratedoverflow fraction containing substantially all of the noncausticizablesalts.

4. A process for clarifying viscous concentrated aqueous liquorscontaining slimy, gelatinous, nely divided solids of different specificgravities and particle size, and for fractionating -the solids into twoportions, one of relatively larger particle size, higher specificgravity solids and the other of relatively smaller particle size rangingto the colloidal state, lower specific gravity solids, the aqueousliquor having a density substantially less than that of the solids ofleast speciic gravity, which comprises mechanically separatingsubstantially all of the solids from the liquor by high speed finecentrifugal clarication, subjecting the thickened solids underow slurryof the ne centrifuging to an intermediate speed coarse 14 centrifugalmechanical separation to produce a more concentrated underflow fractionof solids consisting substantially entirely of the relatively higherspecic gravity and relatively larger particle size, and a lessconcentrated solids overflow slurry containing a major portion of thesolids of relatively lower specific gravity and relatively smallerparticle size and subjecting the overflow from the coarse centrifugalseparation to high speed ne mechanical centrifuging to substantiallycompletely clarify the liquor and to recover a more concentrated slurryof solids of relatively lower specic gravity in relatively smallerparticle size.

5. A process for clarifying viscous concentrated aqueous liquorscontaining slimy, gelatinous, finely divided solids of two differentchemical compounds, the finely divided solids of one of said compoundshaving relatively larger particle sizes and higher speciic gravity, thenely divided solids of the other of said compounds having relativelysmaller particle sizes ranging to the colloidal state and lower specificgravity, and for selective separation of the nely divided solids fromeach other and from the liquor, which comprises mechanically separatingsubstantially all of the solids from the liquor by high speed finecentrifugal clarication, subjecting the thickened solids underow slurryof the fine centrifuging to an intermediate speed coarse centrifugalmechanical separation to produce a more concentrated underflow fractionof solids which is substantially all of the said larger particle sizechemical compound and a less concentrated solids slurry as the overflowfraction containing the predominant portion of the said smaller particlesize chemical compound present in the underflow slurry from the necentrifuging, and subjecting the overflow from the coarse centrifugalseparation to a high speed line mechanical centrifuging to substantiallycompletely clarify the liquor and to recover a more concentrated slurryof the said smaller particle size chemical compound.

References Cited in the le of this patent UNITED STATES PATENTS2,085,538 Lyons .T-une 29, 1937 2,280,998 Brown Apr. 2S, 1942 2,522,605Cundiff Sept. 19, 1950 2,543,281 Perrin Feb. 27, 1951 2,557,629 Boivinetlune 19, 1951 2,596,616 Strezynski May 13, 1952 2,614,110 Davis Oct. 14,1952 UNITED STATES PATENT OFFICE CERTIFICATE OF COIRRIEICTIOLI PatentNol 2,889,982 June 9, 1959 John L., Porter It is here`xnr certified thaterror appears in the-printed specification of the above vnumbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

(SEAL) Attest: ERNEST W. SWIDER XMXXXXXWX Attesting Officer ARTHUR W.CROCKER.

Acting Commissioner of Patents

1. A PROCESS FOR CLARIFICATION OF VISCOUS CONCENTRATED CAUSTIC LIQUORSCONTAINING FINELY DIVIDED SOLIDS OF DIFFERENT SPECIFIC GRAVITIES ANDPARTICLE SIZE AND FOR FRACTIONATING THE SOLIDS CONTENT, THE LIQUIDHAVING A DENSITY SUBSTANTIALLY LESS THAN THAT OF SOLID OF LEAST SPECIFICGRAVITY, WHCIH COMPRISES MECHANICALLY SEPARATING SUBSTANTIALLY ALL OFTHE SOLIDS FROM THE LIQUOR BY HIGH SPEED FINE CENTRIFUGAL CLARIFICATION,SUBJECTING THE THICKENED SOLIDS UNDERFLOW SLURRY OF THE FINECENTRIFUGING TO AN INTERMEDIATE SPEED COARSE CENTRIFUGAL MECHANICALSEPARATION TO PRODUCE A MORE CONCERNTRATED UNDERFLOW FRACTION OF SOLIDSCONSISTING SUBSTANTIALLY ENTIRELY OF THE RELATIVELY HIGHER